D-alanyl–D-alanine terminus of lipid II (D-Ala-D-Ala terminus)

The **D-alanyl–D-alanine terminus of lipid II** is a highly conserved dipeptide sequence at the C-terminus of the pentapeptide stem in the cell wall precursor lipid II, an essential intermediate for bacterial peptidoglycan biosynthesis. Lipid II is composed of a bactoprenol carrier, a disaccharide (N-acetylmuramic acid and N-acetylglucosamine), and a pentapeptide ending in D-Ala-D-Ala[1][2][3][5]. The D-Ala-D-Ala motif forms the classic binding site for glycopeptide antibiotics like vancomycin and oritavancin, which recognize and bind this terminus via a network of hydrogen bonds, sterically blocking further cell wall synthesis and leading to bacterial cell death[3][4][5]. The accessibility of this motif in the membrane makes it a prime therapeutic target, especially in Gram-positive bacteria where the cell wall is pivotal. Alterations at this terminus, such as replacement with D-Ala-D-Lac, are the molecular basis of resistance to glycopeptide antibiotics and are clinically significant in vancomycin-resistant enterococci (VRE) and resistant *Staphylococcus aureus*[6]. The motif is not a classical receptor, enzyme, or channel, but it serves as a critical molecular target for several classes of antibiotics and is essential for bacterial survival. Resistance mechanisms, immune modulation roles, and variable drug affinities define its importance in infectious disease management, biomarker development, and the evolution of antibiotic therapies[5][6][7].

Other names

D-Ala-D-Ala terminusD-alanyl–D-alanine terminusPeptidoglycan precursor D-Ala-D-AlaLipid II D-Ala-D-Ala tail

Binding of glycopeptide antibiotics (e.g., vancomycin, oritavancin) to the D-Ala-D-Ala terminus of lipid II blocks peptidoglycan biosynthesis by forming a stable complex that prevents cell wall assembly; antibiotics may inhibit enzymes (transglycosylases, transpeptidases) required for cell wall formation or form pores/membrane disruption[3][4][5]. Some antibiotics (e.g., lantibiotics) bind to the pyrophosphate moiety of lipid II[5]. Substitution of D-Ala-D-Ala with D-Ala-D-Lac or D-Ala-D-Ser decreases binding affinity and confers resistance to vancomycin[6].